WASHINGTON - NASA scientists on Wednesday briefed reporters on the last days of the Cassini spacecraft that ends its 13-year mission to Saturn in mid-September, transmitting data until the final moment before it plunges into the ringed planet's atmosphere.

Cassini, the first spacecraft to orbit Saturn, will make the last of 22 farewell dives between the planet's rings and surface on Sept. 15. The spacecraft will then burn up as it heads straight into the gas giant's crushing atmosphere.

Cassini's final dive will end a mission that provided groundbreaking discoveries that included seasonal changes on Saturn, the moon Titan's resemblance to a primordial Earth, and a global ocean on the moon Enceladus with ice plumes spouting from its surface.

By destroying the spacecraft, NASA will ensure that any hitchhiking Earth microbes still alive on Cassini will not contaminate the moons for future study.

"We won't watch Cassini burn up. What we'll watch it is slowly turn away from us and we'll watch the indicator on the radio science displays that will go down flat and essentially lost signal. The mission will be over within a minute later," Cassini program manager Earl Maize said.

Cassini's final photo as it heads into Saturn's atmosphere will likely be of propellers, or gaps in the rings caused by moonlets, scientists said.

The spacecraft will provide near real-time data on the atmosphere until it loses contact with Earth at 4:54 a.m. PDT (1154 GMT) on Sept. 15, the National Aeronautics and Space Administration said.

Cassini has been probing Saturn, the sixth planet from the sun, and its entourage of 62 known moons since July 2004. It has provided enough data for almost 4,000 scientific papers.

Since the craft is running low on fuel, NASA is crashing it into Saturn to avoid any chance Cassini could someday collide with Titan, Enceladus or any other moon that has the potential to support indigenous microbial life.